1. Field of the Invention
The present invention relates to an electroluminescence (to be referred to as an EL hereinafter) device which can be used to display characters or graphic patterns and, more particularly, to a thin film-powder hybrid type EL device.
2. Description of the Prior Art
An EL display using an EL device can display characters or graphic patterns with high display quality and therefore is one of flat displays which have been rapidly, widely spread as a terminal of a portable type computer or a terminal of a work station in recent years.
The EL devices are classified into an AC thin film type EL device having a structure in which a thin-film luminescent layer and insulating layers arranged at two sides of the luminescent layer are sandwiched by electrodes, and a DC powder type EL device having a structure in which a luminescent layer consisting of a zinc sulfide powder and a current-limiting layer consisting of a Cu-coated zinc sulfide powder are sandwiched by electrodes. These two types are already put into practical use. In recent years, however, in addition to the above two types of EL devices, a thin film-powder hybrid type EL device (to be referred to as a hybrid type EL device herinafter) having a combination of a thin-film luminescent layer and a current-limiting layer using a powder is proposed as a high-cost performance EL device which can realize high display quality with low cost (e.g., GB2176340 and GB2176341).
FIG. 4 is a sectional view showing a basic arrangement of the hybrid type EL device. A basic structure, a manufacturing method, and an operation mechanism of the hybrid type EL device will be described below with reference to FIG. 4.
A film of a transparent electrode material such as ITO is formed as a transparent electrode 2 on a glass substrate 1 by sputtering or a vacuum vapor deposition method and patterned into a predetermined shape by using, e.g., photolithography. A luminescent layer 3 is formed on the transparent electrode 2 by a vacuum vapor deposition method, a sputtering method, an MOCVD method or the like. A material which is often used as the material of the luminescent layer 3 is obtained by doping, as a luminescent center, a transition metal such as Mn and Cu, a rare-earth metal such as Tb, Sm, Dy, Eu and Ce or a fluoride or chloride thereof into a Group II-VI compound or Group IIa-VIb compound such as ZnS, ZnSe, CaS and SrS. Subsequently, a current-limiting layer 4 is formed on the luminescent layer 3. The current-limiting layer 4 serves as a resistor for preventing an excessive current from flowing through the luminescent layer 3. The current-limiting layer 4 normally consists of a film formed by using a conductive fine powder having a resistivity of 3.times.10.sup.3 .OMEGA..multidot.cm to 1.times.10.sup.6 .OMEGA..multidot.cm and a binder resin by a spray method and having a film thickness of 1 to 30 .mu.m, and preferably, 5 to 30 .mu.m. Examples of the conductive fine powder are Cu-coated ZnS, MnO.sub.2, PbS, CuO, PbO, Tb.sub.4 O.sub.7, Eu.sub.2 O.sub.3, PrO.sub.2, carbon and barium titanate. These compounds are used singly or in the form of mixtures. In order to increase contrast, a black or dark substance is preferably used (however, the substance need not be black or dark). A film consisting of Al or the like is formed as a backplate 5 to have a film thickness of about 1 .mu.m on the current-limiting layer 4 by using a vacuum vapor deposition method or the like. The backplate 5 is mechanically scribed by using a diamond needle, thereby completing a dot-matrix type or segment type hybrid EL device.
Driving is normally performed by applying a DC pulse voltage from a driving power source 9 by using the transparent electrode 2 as an anode and the backplate 5 as a cathode. Alternatively, the device can be driven by an AC voltage. In a dot-matrix type device capable of displaying characters or graphic patterns, a time-division driving method of sequentially scanning lines along the row direction is used. Electrons are injected from an interface between the current-limiting layer and the luminescent layer into the luminescent layer. These electrons are accelerated by a high electric field in the luminescent layer and are bombarded against luminescent centers in a high-energy state. Then, the excited luminescent centers emit light when they are relaxed.
A hybrid type EL device having a structure similar to the above basic hybrid type EL structure is known. For example, a hybrid type EL device in which a dark thin film layer is inserted between the luminescent layer 3 and the current-limiting layer 4 shown in FIG. 4 is reported (e.g., U.S. Pat. No. 4,672,364 and GB2176341A). Since the dark thin film layer is inserted, light emitted from the luminescent layer toward a backplate is absorbed by this thin film layer. As a result, since the light is prevented from being irregularly reflected by the current-limiting layer, the contrast of display can be increased. Especially when a material which is not dark such as a Cu-coated zinc sulfide powder is used as the current-limiting layer, a significant effect can be obtained in an improvement in contrast by inserting a dark thin film layer. Examples of the material of the dark thin film layer are ZnTe (dark brown), CdTe (black), CdSe (black/brown), chalcogenide glass (black), Sb.sub.2 S.sub.3 (black/brown), and other arbitrary dark materials such as oxides and sulfides of transition metals and rare-earth metals, e.g., PbS, PbO, CuO, MnO.sub.2, Tb.sub.4 O.sub.7, Eu.sub.2 O.sub.3, PrO.sub.2 and Ce.sub.2 S.sub.3. The film thickness of the thin film layer is normally 2 .mu.m or less.
In the hybrid EL device having the conventional basic structure as shown in FIG. 4, when Mn-doped zinc sulfide is used for the luminescent layer, a ratio (luminous efficiency) of luminescent energy of the device to energy applied to the device is 0.02% W/W to 0.05% W/W.
In the conventional hybrid EL device in which the dark thin film layer is inserted between the luminescent layer and the current-limiting layer as described above, a luminous efficiency of the device is decreased to be smaller than that of the device having no dark thin film layer.
When the above hybrid type EL devices are used as a dot-matrix type display for displaying characters or graphic patterns, even if a luminous efficiency of the device is 0.05% W/W which is the highest luminous efficiency obtained by the above conventional devices, this luminous efficiency is still unsatisfactory.
If the above hybrid EL devices are used as a display having a small or middle capacity of about 640 .times.200 dots, a luminance of 50 cd/m.sup.2 which is a practical luminance of a display can be obtained by the luminous efficiency described above. If, however, the above devices are used as a display having a middle or large capacity of about 640.times.400 dots or 1,024.times.800 dots, which is currently mainly used, a voltage application time per device, i.e., a so-called duty ratio is decreased. As a result, a luminance is decreased to about 20 cd/m.sup.2 to 40 cd/m.sup.2 which are practically unsatisfactory.
Consumption power of a display is in inverse proportion to a luminous efficiency. When the above hybrid EL devices are used as a display having a small or middle capacity of about 640.times.200 dots with an A5-size panel area, the consumption power of the hybrid EL devices is about 25 W during entire surface light emission while it is about 10 W in the same panel when, e.g., AC thin film EL devices are used. That is, the consumption power of the hybrid EL device is very high.
Since the consumption power of the device is very high, power to be applied to the device is increased to shorten the life of the device.
In the hybrid EL device as shown in FIG. 4, the current-limiting layer 4 prevents the resistivity of the luminescent layer 3 from being decreased to flow an excessive current through the EL device, thereby preventing thermal destruction of the device.
As the resistance of the current-limiting layer 4 is increased, stability of the device with respect to destruction is improved. If, however, the resistance is too high, a voltage drop in the current-limiting layer 4 is increased to increase a drive voltage of the EL device. Therefore, the value of the resistance is limited. When the film thickness of the current-limiting layer 4 is 5 .mu.m to 30 .mu.m, the current-limiting layer 4 preferably has a resistance of 10 to 2,000 .OMEGA. per unit area (1 cm.sup.2) in a direction of film thickness, i.e., has a resistivity of about 1.times.10.sup.4 .OMEGA..multidot.cm to 2.times.10.sup.6 .OMEGA..multidot.cm.
Since the material of the conductive fine powder described above must have the above resistivity after it is fixed by a binder, it desirably has a resistivity of about 1.times.10.sup.4 .OMEGA..multidot.cm to 2.times.10.sup.6 .OMEGA..multidot.cm.
In an initial stage of development of the above hybrid type EL device, a Cu-coated ZnS powder which is conventionally used in a powder type EL device is often used as the material of the conductive fine powder. Recently, however, an MnO.sub.2 powder is used which increases display contrast because it is black and does not change its resistance over time due to no movement of Cu.
These powders are prepared by mechanically pulverizing or milling coarse powders or tabular materials having a comparatively large particle size produced by a precipitation or electrolytic process.
In the above conventional hybrid type EL device, however, a luminance variation is produced during an operation or a life of the device is shortened.
In addition, in the above conventional hybrid type EL device, a luminous efficiency is as low as at most about 0.1 lm/W. Therefore, this conventional hybrid type EL device cannot provide brightness suitable for a practical use.